1. Field of the Invention
The invention relates to radar systems and more particularly to development of high resolution radar scene profiles (i) using a lower rate sampling of a radar wave sequence than previous techniques and (ii) eliminating the pulse compression (matched filtering) as a separate signal processing step in the receive signal chain.
2. Brief Description of the Related Art
In High Range Resolution (HRR) radar, Synthetic Aperture Radar (SAR), inverse SAR (ISAR), and similar systems, an electromagnetic pulse is transmitted and received by radar antennas. The wave sequence that is received by the receiving antenna is typically converted into a sampled digital representation for further signal processing in a digital computer. See M. Skolnik, “Radar Handbook”, McGraw-Hill, 1970; F. Le Chavalier, “Principles of Radar and Sonar Signal Processing”, 2002; and N. Levanon and E. Mozeson, “Radar Signals”, 2004. The analog-to-digital (A/D) conversion takes place either before or after pulse compression. In most modern SAR radar systems, pulse compression takes place after A/D conversion in the computer.
Pulse compression (see M. H. Carpentier, “Pulse Compression and equivalent technologies”, in: Advanced Radar Techniques and Systems, ed. G. Galati, IEE radar, sonar, navigation and avionics series 4, IEE, 1993) is required in order to generate a radar signal that provides a high resolution image of the target that is being probed. In order to obtain an adequate signal back from the target, i.e., to obtain the desired resolution, a short duration pulse is desired. However, short duration pulses are very difficult to implement due to their high power, and therefore another method, known as pulse compression, is used. In pulse compression radar systems on transmit a long pulse with some phase or frequency modulation is generated. On receive the echo wave sequence is correlated with the modulation of the emitted signal, resulting in accurate ranging. The process of correlating an unknown signal with a known signal or a template thereof to detect the presence of the known signal or template is known as matched filtering. Matched filtering for radar pulse compression is equivalent to convolving the received wave sequence with a time-reversed replica of the transmitted radar wave form. The key property of the transmitted radar waveform, be it a chirp or PN sequence, is that its autocorrelation (convolution with a time-reversed version of itself) is close to a Dirac delta pulse that is precisely localized in time.
Often a linear FM chirp signal signal is used, other possibilities include binary phase coded waveforms such as Barker codes and pseudorandom or pseudonoise (PN) sequences. See M. Skolnik, “Radar Handbook”, McGraw-Hill, 1970.
In order to perform adequate A/D conversion of a short duration pulse (which is equivalent to a wideband transmitted chirp signal) high sampling rate A/D converters with large dynamic range must be used. Thus, currently available A/D converter technologies impose a limitation on the resolution of radar systems, because of limitations in the capacity to sample very wideband signals with sufficiently large dynamic range.